The present invention relates to a display and image capture apparatus which is used in visual telecommunication services such as videophones and teleconference services.
With recent developments of telecommunication technology such as video coding techniques, a two-way visual telecommunication system which links remote places through a video and audio communication network, such as a videophone or video teleconferencing system, has rapidly come into wide use. It is now being expected that the expansion of an integrated services digital network (ISDN) and the broadening of the transmission network therefor will make it possible to provide high-quality images which give callers the feeling of actually holding a face-to-face conversation.
FIG. 1 shows an ordinary display and image capture apparatus which is now used in the two-way visual telecommunication. In this apparatus a videocamera 20 is mounted on the top or one side of a CRT display or similar display 10 to capture the image of a user M. In the case of using such an apparatus, the user M is usually looking at the display 10 on which the image of the person he is talking to is being displayed, and hence he will not turn his gaze on the videocamera 20. Thus the eyes of the user M whose image is being displayed on a display of the receiving side at a remote place are not directed to the front--this results in a defect that the conversation between the callers is unnatural because they cannot make eye-contact with each other while speaking.
To offer a small and low-cost display and image capture apparatus which allows callers to look each other in the eyes, the inventor of this application has proposed an arrangement in which a half-transparent mirror, formed by a plurality of micro HMs with their reflecting surfaces inclined, are disposed in front of the display screen and the image of the user reflected by the half-transparent mirror is captured (Kuriki et al U.S. Pat. No. 5,317,405 issued May 31, 1994, corresponding to Japanese Patent Application Laid-Open No. 150684/92). FIGS. 2A and 2B show, in cross-section, two examples of such a conventional half-transparent mirror 3 comprised of micro HMs 3M and FIG. 3 shows the layout of a display and image capture apparatus employing the half-transparent mirror 3 depicted in FIG. 2A. The half-transparent mirror 3 is formed by an array of a number of strip-like micro HMs 3M with their reflecting surfaces inclined. That is, ridges 3R, saw-tooth in cross-section, are formed in parallel over the entire area of one surface of, for instance, a transparent plastic sheet 3B and thin half-transparent reflecting layers of aluminum or similar metal are coated on the slopes of the ridges 3R to form the micro HMs 3M. In FIGS. 2A and 2B the reference character .theta. indicates the angle of inclination of each micro HM 3M.
The half-transparent mirror 3 of such a construction as mentioned above is mounted on the front of the display 10. The videocamera 20 is placed in the direction in which light incident to the half-transparent mirror 3 from the front is reflected. Consequently, the user M is allowed to see the display screen of the display 10 through the half-transparent mirror 3 and a portion of the incident light is reflected in the direction of an angle 2.theta. as shown in FIG. 3; thus, the videocamera 20 disposed in that direction captures the image of the user M viewed from the front, enabling eye contact between callers.
However, since the ridges 3R covered with the micro HMs 3M in the conventional display and image capture apparatus each act as a prism to light transmitted through the half-transparent mirror 3, light from the display screen is bent downward in such a layout as shown in FIG. 3, posing a problem that the brightness of the lower part of the display screen decreases as the lower marginal edge of the screen is approached. This problem is not so serious in the case of a desk-top display because its display screen is relatively small, say, about 10 inches at the largest, but in the case of using a display whose screen size is in the range of from tens to hundreds of inches, such as a projector, the brightness at the middle and lower parts of the screen decreases substantially.
In the half-transparent mirror 3 of FIG. 2A the micro HMs 3M each serve both as a micro reflecting surface and a micro transmitting surface and reflect the image of the user M to the videocamera 20 while at the same time they permit the passage therethrough of the display image from the screen. In the half-transparent mirror of FIG. 2B each ridge 3R has two sloped surfaces 3M and 3T, the one sloped surface 3M is given a reflective coating for reflecting an image to the videocamera 20 and the other sloped surface 3T is used to emit transmitted light from the display screen. This half-transparent mirror can also be used with the display and image capture apparatus. The transmissivity of the micro reflecting surface 3M is so low as not to contribute to the observation of the display image, that is, its reflectivity is high. The micro reflecting surface 3M will also hereinafter be referred to as a micro HM as in the case of FIG. 2A. Also in the case of using the half-transparent mirror 3 depicted in FIG. 2B, transmitted light which is emitted from the micro transmitting surface 3T is bent upward by the prism effect of the ridge 3R as shown.
In FIG. 11 there is shown by white squares the brightness distribution of the display screen from its top to bottom area at its central part. As will be seen from FIG. 11, the brightness decreases toward the lower marginal edge of the display screen from its central portion, the brightness at the center of the screen at which the user looks most frequently is lower about 20% than the maximum brightness, and at the lower part the brightness is lower as much as 70% or more than the maximum value. For this reason, the display and image capture apparatus shown in FIG. 3 cannot be applied intact to a teleconference system or the like that calls for a large display screen.